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Ground Fault Detection transformer sizing

Ground Fault Detection transformer sizing

Ground Fault Detection transformer sizing

(OP)
I have a substation with a transformer having a 34.5KV ungrounded tertiary winding which supplies a wye-connected reactor and a station aux transformer.  I am planning to design a ground fault detection system using a grounded-wye to broken delta transformer.  Everything I have read indicates that the ground detection transformer and the resistor which is placed across the broken delta should be sized based upon the system charging current in order in prevent ferroresonance and other issues.  However, the primary means of determining the system charging current appears to be either measuring it or using previous experience.  The little real-world data I have seen published generally goes no higher than 13.8KV and is mainly derived from medium-voltage distribution systems.  I think it is reasonable to expect that the system charging current in the system I am describing will be very low, but I'd like to have some sort of estimate based in reality. :) Does anyone have any useful information or a good reference?

RE: Ground Fault Detection transformer sizing

Use three distribution transformers for your ground detection. That will avoid possible "phantom delta' issues with a three legged three phase transformer.
For a 5 Amp ground fault threshold I would guess 167 KVA or 250 KVA, depending on how fast you will clear a fault.
You won't have much charging current for station service and a reactor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Ground Fault Detection transformer sizing

Your system is 34.5kV ungrounded. If you want to change for high impedance grounded, middle impedance grounded , etc you must use distribution transformer and so on.

If you want to keep it ungrounded, you must use Potential Transformer
with secondary open delta with resistor , to install 59N to detect fault-to-ground.
 See Bassler guide to calculate resistor.

RE: Ground Fault Detection transformer sizing

(OP)
How did you arrive at 5A as the desired fault current? And how does that equate to the transformer size you have chosen? Are you simply multiplying square root of 3 times line volts times current? Is that valid? It's a much larger transformer than I expected.

As for ungrounded with ground fault detection vs high impedance grounded, is there a practical difference in the system described? I have read the Basler guide, and they make certain assumptions without explaining why. For example, the transformer size.  

RE: Ground Fault Detection transformer sizing

Your is system heas been operating as  34.5kV ungrounded. why do you want to change it? Is there any recommendation to change?
Why do you worry about 5A? Your objective is only  to detect fault-to-ground.
  

RE: Ground Fault Detection transformer sizing

(OP)
No, it hasn't been operating this way. It doesn't exist yet. I want to understand why I would make one choice over another. It doesn't help me to give me an answer without explaining how you arrived at that point.  

RE: Ground Fault Detection transformer sizing

Your system is not ready. Now is different! How about your 34.5kV system:
1- Are there loads beside  reactor and  station aux transformer?
2- your SAT is 34.5/ low voltage and delta HV/ Yn low voltage?
3- 34.5kV system is a generating busbar?
4- How is the unifilar?
I have seen ungrounded system (0 A),grounded by high-impedance (5-30A),  grounded by middle impedance(100-600A), grounded by low impedance (1F < =  3F).
 

RE: Ground Fault Detection transformer sizing

JCGreff,
Your intention is to install a ground fault detection system on a DELTA connected network using WYE-BROKENDELTA transformer.The estimation of charging current is needed if your system is a high resistance grounded system so that your NGR is designed to allow little more than 3xcharging current of the network.But here your network is DELTA ungrounded.Therefore,what you need is three VTs each having maximum 500VA (very typical)connected primary in WYEGND and the secondary in BROKENDELTA across a simple voltage relay(ANSI function 59N).BASLER model BE1-59N is very widely used low cost relay.The resister connected accross this voltage relay is nothing to do with the charging current because it is the stabilizing resister to mitigate any possible ferroresonance.The reason for saying that is whether you connect or not this resister,the relay will sense the zero sequence voltage during a ground fault and it will alarm or trip depending on its settings.Therefore,first calculate the maximum possible voltage during a ground fault accross the stbilizing resister considering the turns ratio of the VTs and then size the resister for the full thermal rating of the VT.Donot oversize the resister because it may "invite" ferroresonance.

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